When a gasoline engine decelerates, if there is undershoot of rotation speed, an engine stall or rotational fluctuation of the engine is apt to occur. In order to suppress such an undershoot of rotation speed during deceleration, Tokkai Hei 6-288327 published by the Japanese Patent Office in 1994 discloses a method of correcting an intake air quantity and ignition timing of the engine during deceleration.
According to this prior art, fuel supply is cut during engine deceleration, and when the engine rotation speed has reached a predetermined value in the vicinity of the idle rotation speed, the fuel supply resumes and the engine then rotates at the idle rotation speed. When the fuel supply resumes, the air supply amount to the engine is increased by a constant amount. In addition, an ignition timing is corrected according to a difference between a target rotation speed and a real rotation speed of the engine.
However, the engine deceleration state may be gradual deceleration or sudden deceleration, i.e. it is not constant. Moreover, an intake negative pressure in the intake manifold when the engine decelerates to the idle state is also different according to the engine rotation speed before deceleration, deceleration rate and air supply amount during deceleration, etc.
Simply increasing the air volume by a constant amount is therefore not always sufficient to effectively suppress the rotational fluctuation of the engine rotation speed, even when it is accompanied by the ignition timing control.